Eocene Period, 58-36 million years B.P. In a rectangular matrix. 385 grams, 16.5 x 13.5 cm (6 1/2 x 5 1/4 in.). Lincolnshire, UK, collection. Seasonally, some 56 million years ago, in the Green River Region of Wyoming, torrential rains fell on the mountains. The rain flowed down the mountain sides, forming wide shallow lakes in the valley below. During the dry season, these lakes evaporated (similar to certain parts of Africa today). Millions of fish died in this short period of time. This climatic cycle happened over a 20-thousand-year period. Layer upon layer of mud was deposited over these years, entombing huge numbers of fish. At a much later date volcanic activity uplifted this area to expose their fossils.